1. Field of the Invention
This invention relates to electrophotographic apparatus and more particularly to improvements in such apparatus which avoid the blurring effects caused when such apparatus with heated photosensitive elements are taken out of use for substantial periods of time.
2. Description of the Prior Art
As shown in FIG. 2, a known electrophotographic apparatus has a latent image forming device, a developing device, a cleaning device, etc., as major structural elements arranged around a photosensitive drum 1. The outer surface of the photosensitive drum 1 rotates in the direction of an arrow A and is electrostatically charged in a uniform manner by a charger 2. A light image from an original to be copied is formed on charged surface of the photosensitive drum 1 through an exposure slit 3 and a not shown lighting and optical system. This produces an electrostatic latent image on the surface of the photosensitive drum 1. A toner image is then formed on the electrostatic image by a developing device 4a or 4b. Subsequently a recording material, such as a paper sheet, is sent by a paper supply device (not shown) and a pair of register rollers 5 to the surface of the photosensitive drum 1. The toner image is then transferred onto the recording material from the surface of the photosensitive drum by transfer and separation devices 6. The recording material is then sent to a fixing device (not shown) by a transport system 7. There the toner is fixed on the recording material. In the meantime, the remaining toner left on the surface of the photosensitive drum 1 after transfer is removed from the drum by a cleaning device 8. Also, the remaining electrostatic charge is erased from the drum by an eraser lamp 9 so that the drum can be used repeatedly.
The photosensitive drum may be the OPC (organic photoconductor) type, which is widely used, or it may be the "asi" (amorphous-silicon) type which is also used and which has excellent durability.
Within the photosensitive drum 1 there is provided a heater (not shown) which warms the drum surface to a certain temperature for the purpose of increasing photosensitivity or preventing blurring of images which would otherwise be caused by moisture absorption on the photosensitive drum surface. Especially for the purpose of preventing blurring of images it is necessary to apply an electric current to the heater independently to keep the drum surface warmed to a temperature of about 40-44 degrees centigrade, not only during normal operation but also when the apparatus is shut off. That is, when the power switch (main switch) for the electric power supply which drives the means for copying operation such as the charger 2, the lighting and optical system, the developing devices 4a and 4b, the rollers 5, the charger 6, and the lamp 9, is turned off and the whole apparatus is left suspended, for example overnight, the heater which warms the drum surface is left on.
However when the photosensitive drum is kept warm during night or other extended idle periods for the purpose of preventing blurring of images as aforementioned, a problem arises in that the density of images produced by the apparatus is low at the time of switching on the apparatus (at the time of turning on the power switch), for example first thing in the morning. The precise cause of this is unknown; however it is presumed that owing to the suspension of machine operation for a long time, such as suspension during night hours, the toner on the developing roller is influenced by the heat of the drum heater and deteriorates. Therefore the state of the triboelectricity of the toner on the developing roller becomes unstable when the toner is re-used. Thus, it is believed that such tainted image is caused by the toner's deterioration.